Conventional toilet bowl cleaners for home or commercial use are typically composed primarily of sodium bisulfate with minor amounts of detergent, fragrance, and corrosion inhibitor. These cleaners optionally contain a small amount of a strong oxidant such as OXONE.RTM. monopersulfate compound and serve the function of soil and stain removal.
Conventional toilet bowl cleaners may take the form of: tablets that are placed in the bottom of urinals or tanks; liquids that are dispensed directly into toilet bowl or urinal water; and gels or cakes that are hung over the side of a toilet bowl or tank and that slowly dissolve when in contact with flush or tank water.
In spacecraft urinal systems, where urine is collected and stored over long periods of time, the pretreatment of urine is of eminent importance. It has been found that pretreatment serves to increase the reliability of such systems and serves to reduce the amount of maintenance time for cleaning and repairing system hardware. In particular, pretreatment of urine is required in spacecraft urinal systems to control odors, fix urea, and control microbial growth. In addition, pretreatment is required to reduce or eliminate fouling of the hardware and plumbing with urine precipitates and thereby assure long term use of urine collection equipment.
Typically urine contains about 96 percent water and about 4 percent solids in solution. About half of the solids consist of urea, (NH.sub.2).sub.2 CO, while the remainder of the solids include chloride, sodium, potassium, nitrogen, ketosteroids, phosphate, sulfur, ammonia, creatinine, and uric acid. Without some form of pretreatment, many of the constituents of urine will decompose at room temperature and will become contaminated with bacteria, which will result in further decomposition. In particular, urea decomposes resulting in the formation of ammonia and carbon dioxide, and a significant amount of solids precipitate out of solution resulting in the fouling of urine collection equipment. Additionally, the evaporation of water in urine, resulting from air entrainment therein, facilitates the precipitation of urine salts.
The pretreatment of urine for long term storage in micro-gravity environments, such as those found in spacecraft urinal systems, present numerous technical hurdles however. Spacecraft urine collection equipment or devices employ little or no flush water. Chemicals, in powder form, are not easily metered into a fluid stream in such micro-gravity environments and many pretreatment chemicals in solution degrade over time. Moreover, weight and volume penalties associated with any pretreatment scheme for spacecraft applications must be minimized.
Known methods of pretreating urine in such microgravity environments include: mixing solid OXONE.RTM. and liquid sulfuric acid with an aqueous medium for liquid injection into a urine stream; and dispensing water soluble organic acids in powder or tablet form, via mechanical means.
Such pretreatment methods are presently employed only on extended duration (e.g., up to 30 days) and long duration (e.g., greater than 30 days) space flights. Present experience has revealed however that pretreatment is necessary on short duration (e.g., up to 2 weeks) flights as well. In particular, during a recent Shuttle flight, significant deposits of urine solids were discovered upstream of a liquid collection storage tank. It was recognized that these deposits can lead to premature failure of the urinal system.
Known micro-gravity pretreatment methods are problematic however in that the use of liquid sulfuric acid presents severe handling problems which include containment and corrosion problems. In addition the efficiency of OXONE.RTM. in solution degrades significantly over time. Moreover, dispensing methods presently require dual injection systems (i.e., mixing and injecting one portion of the pretreatment chemicals upstream of a urine separator device and the other portion downstream of the device) which contribute to complexity, increased maintenance and reliability concerns.
Further to the above, Hasting et al. U.S. Pat. No. 5,328,633 discloses soluble, extended-release tablets for removing and preventing plaque formation that are placed into a system, such as a ship's collection, holding, and transfer system. Once in the system, the tablets maintain the pH of the system at an acidic state that reportedly prevents the precipitation and subsequent build up of a plaque of insoluble salts and that reportedly dissolves existing plaque. The tablets are formed by compression molding techniques (see, Column 4, lines 55 to 68) and are made up of: 50 to 95 wt % of a benign organic acidic agent (e.g., citric acid); and 5 to 50 wt % of an extended-release binder comprised of poly(ethylene glycol) (MW=4,000 to 20,000 grams/mole) and poly(ethylene oxide) (MW=100,000 to 5,000,000 grams/mole). However, Examples 1 to 17 of Hasting et al. use binder concentrations of 25 to 30 wt %, which would contribute to weight and volume penalties in spacecraft applications. Moreover, it is submitted that binder concentrations of less than or equal to 10 wt % would not serve to adequately bind the composition of Hasting et al.
It is therefore an object of the present invention to provide an extended-release chemical formulation in tablet form that requires minimal, if any, use of a binder component yet is non-dusting, pliable, structurally strong, and not weakened by exposure to aqueous streams.
It is a further object to provide an extended-release chemical formulation that serves to prevent urine solids from precipitating and depositing on internal collection and storage tank surfaces, that aids in the reduction of the transfer of urine odors, and that stabilizes urine for long term storage.
It is yet a further object of the present invention to provide a simple and reliable method for controlled dispensing of an extended-release chemical formulation into a liquid stream that requires little or no maintenance and that avoids the problems associated with mechanical injection means.